The fourth generation, 4G, wireless access within the 3rd generation partnership project, 3GPP, long-term evolution, LTE, is based on orthogonal frequency-division multiplexing, OFDM, in downlink and discrete Fourier transform, DFT, spread OFDM, also known as single carrier frequency division multiple access, SC-FDMA, in uplink. Here, the uplink consists of the physical channels PUSCH, PUCCH, and PRACH and of the physical signals DMRS and SRS. According to the 3GPP specification, see, e.g., 3GPP TS 36.211 V11.3.0, the PUSCH, PUCCH, DMRS, and SRS all use an IFFT of size 2048 in the transmitter, with a sampling rate of 30.72 MHz. The same size of 2048 can be used for the FFT in the receiver. Dedicated hardware is commonly used for these FFTs. With another sampling rate than 30.72 MHz, the IFFT and FFT size will change accordingly.
The Physical Random-Access Channel, i.e., the PRACH, is used for initial access for a wireless device into the radio access network and also for timing offset estimation, i.e., estimation of timing offset between wireless device transmissions and reception at a base station. A description of this procedure is given in 3GPP TS 36.213 V11.3.0. An illustration 100 of PRACH, as specified for LTE, see, e.g., 3GPP TS 36.211 V11.3.0, is given in FIG. 1. Here five different formats, referred to in FIG. 1 as Format 0-Format 4, are specified where a PRACH preamble 101, 101′ consists of one 101 or two 101′ sequences, each of length 24 576 samples. The preambles have a cyclic prefix 102, CP, of length between 3 168 and 21 024 samples for formats 0 to 3.
Several methods have been proposed for how to detect the PRACH preambles transmitted by the UE, see e.g., S. Sesia. I. Toufik. M Baker “LTE, The UMTS Long Term Evolution, From Theory to Practice”, Second Edition, John Wiley & Sons Ltd., 2011, where both a full frequency domain and a hybrid time-frequency approach are presented. In the full frequency domain approach the received signal is processed with an FFT corresponding to the length of the preamble. Hence, as shown in FIG. 2, an FFT 203 of length 24 576 is thus required for each antenna. Dedicated hardware is commonly used for this PRACH FFT. After this large FFT, the PRACH bandwidth is extracted, which is a subset of the output from this large FFT.
In the hybrid time-frequency approach, a low-pass filter is first used in the time domain in order to extract the PRACH bandwidth. This lowpass filter is followed by an FFT of a size much smaller than 24 576. However, one such low-pass filter has to be applied to each antenna signal.
Consequently, as illustrated by FIGS. 1 and 2, the PRACH preamble as specified in LTE Release 8 covers a time interval which is much longer than the length of OFDM symbols used for other transmissions such as user data symbols. Current PRACH preamble receivers are thus designed under the assumption that propagation conditions are not varying significantly during the length of the preamble. This may be problematic, since assumptions, or constraints, are placed on the communication system. These constraints include expectations on low UE speed, i.e., Doppler spread, low frequency errors and low Doppler shifts, and also low phase noise in transmitters and receivers.
Thus, there is a need for an improved PRACH signaling technique, i.e., a preamble transmitter and receiver, which does not place or otherwise imply the above mentioned constraints on the communication system.
With currently emerging technologies, such as 5G communication systems, the use of many antenna elements is of great interest. As illustrated in FIG. 3, the antenna signals can come from several antenna polarizations 304. Here, the antenna signals 305 are first received in a Radio Unit, RU, 306. The signals are then sampled and quantized in an Analog-to-Digital Converter, ADC, 307. A transformation from time to frequency domain is done using an FFT module 308, or, alternatively by a DFT module not shown in FIG. 3, after which a PRACH receiver 309 is applied to detect a preamble comprised in the received radio, i.e., antenna, signals. Here, an FFT is typically calculated for each antenna or for each subset of antennas, such that different users and channels in different sub-bands of the received signal can be extracted before further signal processing.
FIG. 3 illustrates current PRACH receivers having multiple antennas. FIG. 3 visualizes that with a large number of receiver antennas 310, the amount of FFT processing in the receiver is also large, which is generally a drawback. With dedicated antenna-signal processing only used for PRACH, a significant amount of special hardware for PRACH must be included, which hardware causes increased material cost as well as increased energy consumption. Also, running PRACH-specific antenna-signal processing consumes power and requires cooling capacity. Consequently, there is a need for a PRACH receiver better suited for multiple-antenna operation.
In order to increase received signal strength, a beamforming procedure can be used in which several antenna signals are scaled, phase shifted, and added before the PRACH receiver 309 is applied. Beamforming aims at combining received signals from several antennas such that more signal energy is received in specific spatial directions. Several beams can be formed in order to beamform towards different spatial directions. With two polarizations, the antenna signals from each polarization are typically beamformed separately. The same, or different, beamforming can be applied to the different polarizations.
This beamforming 411 can be done in the frequency domain, i.e., after the FFT 408, as illustrated in FIG. 4. After the FFT 408, the individual sub-carriers can be extracted such that different physical channels and signals can be extracted. With digital beamforming 411 in the frequency domain, the antenna signals are first processed with an FFT 408 and then beamformed 411. In this manner, different sub-carriers can be beamformed differently. This allows for different beamforming for different physical channels and signals. Also, if several UEs are multiplexed in frequency, then these can be processed with individual beamforming.
However, with digital beamforming, a specific PRACH FFT has to be calculated for each receiver antenna, before extracting the PRACH bandwidth and beamforming into a smaller amount of signals. This is potentially a drawback due to the added signal processing required.
Alternatively, the beamforming can be done in the time domain 511b, as shown in FIG. 5. Here, the beamforming is done on a digital signal, i.e., after analog-to-digital conversion by the ADC 507. However, since the FFT 508 is calculated after the beamforming 511b, all sub-carriers are beamformed in the same spatial direction, which is a potential drawback in some scenarios, e.g., where UEs are spread out over a large area.
An alternative time-domain beamforming 611c is illustrated in FIG. 6, where the beamforming 611c is done before ADC 607. Here, the beamforming is done on an analog signal, i.e., before analog-to-digital conversion by the ADC 607.
Combinations of analog and digital beamforming and time- and frequency-domain beamforming are also possible.
With analog beamforming, such as the beamforming illustrated in FIG. 6, the number of spatial directions for PRACH is limited by the number of analog beamformers. In LTE release 8, the PRACH preamble, and thus also the PRACH FFT, spans almost a whole sub-frame. The analog beamforming must therefore be fixed during a whole sub-frame which limits the number of beamforming directions.
Hence, present solutions for receiving PRACH and performing UE initial access and timing offset estimation are costly in terms of extra hardware and design effort, as well as in increased energy consumption and signal processing resources. Furthermore, improvements in the interworking between PRACH reception and beamforming in multiple antenna systems are preferred in order to reduce complexity of implementation.
It is an object of the present disclosure to provide solutions to, or at least mitigate, the above mentioned deficiencies in the art.